The remarkable development in today's IT technology is based on silicon semiconductors. Semiconductors are not only important in science and researches, but also in industry and economy. However, as development of silicon semiconductors draws near to its limitation in principle, the world has now begun to turn to new-generation semiconductors aiming at diversification of technologies. In this respect, compound semiconductors capable of controlling both light and electrons have been attaining high spotlight.
Gallium nitride (GaN) is the most typical compound semiconductor with a direct band structure and a wide band gap energy (3.45 eV). It is very useful because it is operable in blue and UV regions of light spectrum. However, conventional gallium nitride semiconductors, having diameters of micrometer size and two-dimensional structures, are limited in their uses in electric and electronic nano devices which are becoming smaller and smaller. The use of blue laser in DVD enables much improved integration of data than red laser, but to do so, realization of a nano (a billionth) sized semiconductor is essential. Thus, it is required to fabricate gallium nitride into nanotube or nanowire. Besides, nano-size semiconductors can be widely used in various fields, from nano/molecular electronic engineering to optical/probe scanning spectroscopy, due to their physical, optical and electronic properties.
Nanowire refers to a one-dimensional, linear material of hundreds of nanometers, micrometers or millimeters long, whose length is much larger than its diameter. Physical properties of nanowire depend on its diameter and length. Whereas researches on physical properties of nano particles and manufacture methods thereof are being carried actively, there are few manufacture methods of nanowire available. Examples of conventional methods include template method, chemical vapor deposition and laser ablation.
In the template method, pores of nanometer to hundreds of nanometer units are used as template of nanowire. For example, an aluminum electrode is oxidized to transform the surface into aluminum oxide, which is etched electrochemically to obtain nanopores. The resultant template is immersed in a solution containing metal ions and electricity is applied. Then, the metal ions accumulate on the aluminum electrode, filling the pores. Finally, the oxide is removed to obtain metallic nanowire. However, this method is too complicated and requires a lot of time. Moreover, the obtained nanowire is polycrystalline with poor quality and the process is not suitable for mass production.
Besides, since the diameter and length of the nanowire are determined by the size and depth of the pores, it is very difficult to fabricate long nanowire having a diameter of several nanometers with the present technology.
To solve these problems, chemical vapor deposition and laser ablation have been commonly used. Through these alternatives, nanowire of semiconductors, such as GaN, GaAs, GaP, InAs and InP, has become available.
In chemical vapor deposition (CVD), a source gas containing a component material is injected into a reactor and decomposed by heat or plasma, thereby forming nano-sized tube or wire on the substrate. Chemical vapor deposition is classified into low-pressure CVD (LPCVD), ambient-pressure CVD (APCVD) and high-pressure CVD (HPCVD), depending on the pressure inside the reactor. Plasma enhanced CVP (PECVD) enables fabrication of nanotube at relatively low temperature using plasma. This method requires a process of forming a transition metal film prior to the growth of carbon nanotube. The transition metal catalyzes decomposition of the source gas and acts as nucleus from which nanotube or nanowire grows. In actual manufacture of nano material, the nano material grows on a wafer
In the field of data storage, direct transition type, broadband group III-V semiconductors are drawing attention with regard to metal-based giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR). When a transition metal like manganese (Mn) is doped into these group III-V semiconductors, not only spin transfer but also spin control is achievable. Consequently, semiconductors for operation circuits become available. They are drawing attention as new-generation spintronics materials capable of offering light emitting diode (LED) properties through P-N junction.
Until now, researches mainly focused on three-dimensional films. Since the three-dimensional films are grown by low-temperature molecular beam epitaxy (MBE), the doped GaMnN film could not fully operate its performance because of internal defect and formation of secary phase. Thus, a perfect single crystal without internal defect even when doped with manganese (Mn) was required and development of one-dimensional nanowire capable of realizing quantum effect was needed. And, even if one-dimensional nanowire doped with manganese (Mn) was to be grown, control of the doping concentration of the transition metal was impossible. The doping concentration of manganese (Mn) is an important factor because it greatly affects the magnetization value and the concentration of holes, or carriers.